Various air inlet systems and particularly ram air inlets are known in the field of aircraft construction. These air inlet systems include recessed or recessible air channel inlets that are arranged on various aerodynamic surface areas of the aircraft. These inlets serve to deflect at least a portion of the boundary layer airflow from the exterior of the aircraft during flight thereof, so that the boundary layer air can flow into the interior of the aircraft in a suitable airflow channel, to be used by any one of a number of different systems of the aircraft.
Also, a so-called NACA sink inlet or intake developed by the National Advisory Committee for Aeronautics (NACA) is also known in the art, and is often used in aircraft for various air suctioning systems. Examples of such systems installed in aircraft include the auxiliary power unit (APU) intakes, air conditioning pack ram channel inlets, belly fairing ventilation inlets, and the like. These NACA sink inlets installed typically in aircraft primarily serve to provide exterior air to smaller power plants or auxiliary devices, for example in the form of ram air inlets for providing cool air to the air conditioning packs. Such a NACA sink inlet is also conventionally arranged in the belly fairing of the aircraft of the Airbus family for providing external air for cooling the bleed air extracted from the jet engines in corresponding heat exchangers of the air conditioning systems. In order to provide air to the auxiliary power unit (APU), a NACA sink inlet having parallel side walls is installed in the bottom surface or belly of the fuselage in the area of the horizontal tail surfaces of the aircraft. Additionally, a so-called pack bay ventilation inlet is provided to ventilate the air conditioning pack bay and also for tempering the aircraft structure. This pack bay ventilation inlet is typically arranged in the forward portion of the belly fairing. Moreover, two sink inlets are typically provided on the cowling of an engine, whereby one of the inlets serves to provide cooling air for carrying out the first stage cooling of the bleed air extracted from the engine, and the second inlet provides air for ventilating the intermediate space between the engine and the cowling. Another use or arrangement of such an air inlet is as a wing tank inlet, for providing air for ventilating the fuel tanks and thereby achieving a pressure equalization. In this context, the wing tank inlet is arranged on the bottom exterior surface of the respective wing.
All of the above described different air inlets have an inlet geometry, especially including inlet edges or an inlet lip against which the relative wind flows as the aircraft is in flight, such that this inlet geometry generates a pair of air vortices which transport the energy-rich air of the boundary layer into the inlet. In this manner, during flight of the aircraft, a useable pressure recovery is achieved for the air guide channel system arranged downstream of the air inlet, which serves to provide an air mass flow transport. The amount of turbo-machine or turbine engine suction power that would otherwise be necessary can be considerably reduced or completely eliminated through the use of such air inlets. Namely, the necessary aerodynamic energy of the air supply for the above mentioned auxiliary aggregates or systems of the aircraft is solely provided by the motion of the aircraft through the surrounding air during cruise flight of the aircraft, or more accurately is provided by the aerodynamic flow of the surrounding air relative to the air inlet edges or lips of the NACA sink inlets. However, when the aircraft is operating on the ground, either parked, taxiing or rolling for take-off or landing, this aerodynamic flow of the surrounding air is completely non-existent or is insignificant and therefore does not provide the above described effects. During this period of time of ground operations, the required volume flow of external air is sucked through the respective air inlet such as a NACA sink inlet, for supplying the respective auxiliary system of the aircraft.
Present FIG. 1 shows the airflow situation in connection with a NACA air inlet 1' for the situation when the aircraft A' is flying forward in cruise flight. As shown in FIG. 1, the flow of outside air 5' flows smoothly from left to right, whereby a portion 53' of the outside air is sucked smoothly into the air inlet 1'. The nose portion 6' of the inlet lip 3' faces directly into the incident flow, whereby the airflow is divided into a portion 53' that flows into the air inlet 1' and a portion 54' that continues to flow along the exterior of the aerodynamic surface of the aircraft fuselage.
In comparison to the above described flow condition that exists during forward flight of the aircraft, a completely different flow pattern exists around the air inlet 1' during the ground operations of the aircraft. Such a flow condition and situation is illustrated in present FIG. 1A. In this condition, there is no longer a general flow of the relative wind from left to right along the outer aerodynamic surface of the aircraft. Instead, the outside air 5' is substantially stagnant and is sucked from all directions, i.e. from the left and from the right and from all directions underneath the aircraft, into the air inlet 1' by the suction that prevails in the air guide channel C' connected downstream of the air inlet 1'. As a result, outside air 5' being sucked from the right in FIG. 1A fully flows around the 180.degree. curved or rounded surface of the nose portion 6' of the inlet lip 3' of the air inlet 1'. Largely due to the 180.degree. curve of the airflow pattern of the outside air 5' flowing directly over the nose portion 6' of the inlet lip 3', a flow separation bubble B' is formed directly downstream from the upper nose edge 61' of the nose portion 6' within the air guide channel C'. This separation bubble B' extends into or is sucked into the air intake system connected to the downstream end of the air guide channel C'. This leads to high inlet losses, which in turn reduce the theoretically obtainable air flow and air power that would be achieved without such inlet losses.
A thesis by Michael Klas (Matr. No. 041101, "Theoretische Untersuchung zur Erhohung des Durchsatzes am Staulufkanal der Klimaanlage am Beispiel des Airbus A330/A340" ("Theoretical Investigation for Increasing the Throughput at the Ram Air Channel of the Air Conditioning Plant in the Example of the Airbus A330/A340"), Chapter 5, .sctn.5.2.6.2 "Vorprofil", page 66, Thesis in the Fachhochschule of Aachen, Federal Republic of Germany, August 1996), suggests that the arrangement of a wing-like aerodynamic profile member on the surface of the aircraft in front of the NACA sink inlet will influence flow lines of the air flowing past this aerodynamic profile member. The deflection of flow lines downstream from this profile member makes it possible for the upper energy-rich boundary layer air to flow into the channel inlet. In this manner, in addition to the slight resistance increase caused by the profile member, there should also arise an increase in the ram air pressure recovery ratio. To achieve this, Klas further explains that a particular conclusion regarding the magnitude of the curvature and the advantageous spacing of the profile member relative to the intake for achieving an effective deflection of the external airflow can once again only be determined by means of a wind tunnel study or a computer simulation.
These remarks of Klas are illustrated by FIG. 5.11 which shows the construction of the described air inlet, with an inlet lip against which the incident air flows, a channel inlet that is recessed along its length, and a leading edge profile member that is rigidly arranged in front of or upstream of the sink inlet. FIG. 5.11 of this thesis reference further shows the airflow line pattern of the boundary layer air. In this manner, the thesis suggests a possibility for improving the inlet flow behavior of the free external airflow of the boundary layer into the NACA sink inlet during the cruise flight of an aircraft. However, this thesis reference does not mention or suggest anything about the different flow pattern that exists in the area of the air inlet, and the resulting disadvantageous separation bubble and the like, when an aircraft is parked or operating at low speeds on the ground rather than in cruise flight. The problems associated with the air inlet airflow pattern during ground operation of the aircraft were apparently not recognized, and in any event were not treated, nor were any motivations given in this regard.
U.S. Pat. No. 4,174,083 discloses a conceptual solution for the problem of influencing the airflow as carried out by the Boeing Corporation. According to this reference, a delta-shaped wing with a special tilt angle is moved or projected into the external airflow over the inlet region of an above-described NACA inlet. This delta-shaped wing effectuates a variation of the airflow outside of the inlet, in connection with a maximum opening of the inlet flap. Due to the wing form that is tipped or tilted toward the inlet, the energy-rich free external airflow is deflected toward the rear portion of the inlet. The pair of vortices arising on the wing edges is similarly sucked into the channel inlet so that no additional resistance arises due to the vortices. However, in this prior art arrangement, a very wide open flap setting of the intake flap is required before a pressure increase is achieved due to the deflected airflow entering into the channel inlet. When the flap is closed or even when the flap is partially open over a large opening range, the deflected air will not enter into the channel of the sink inlet. As a result, the pair of vortices will cause increased resistance values due to the vortex formation. Moreover, no details have been disclosed regarding the ram air pressure recovery ratio and the resistance increase values for this single configuration of the arrangement, so that further wind tunnel tests and the like would be necessary to determine the efficiency of such an airflow influencing measure.
Various other measures for influencing the airflow at or near the inlet lip of engine air inlets are known in the art, for example from U.S. Pat. No. 3,652,036 especially in connection with FIGS. 1 to 5 and the associated description thereof, German Patent Laying-Out Publication 1,139,701, especially in connection with FIGS. 1 and 2 and the associated description thereof, and U.S. Pat. No. 3,222,863, especially in connection with FIGS. 1 to 4 and the associated description thereof. However, there are no suggestions or motivations as to whether such inlet lip configurations or auxiliary measures can also be used advantageously in connection with the inlet lip of a NACA inlet.